1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to a mounting arrangement for the die closing unit on the machine base of an injection molding machine in such a way that two longitudinal guide rails on the machine base provide a positive centering relationship between the die closing unit and the machine base.
2. Description of the Prior Art
It is known to arrange longitudinal guide rails on the machine base of an injection molding machine, in order to provide guidance and support for the movable die carrier member of the die closing unit, thereby partially or completely relieving the tie rods of their guidance and weight-supporting function. The advantages of this exterior guidance by means of guide rails are twofold: The guide elements are readily interchangeable, when worn, without the need for removing the movable die carrier member from the machine. The guides are adjustable for precision-alignment of the movable die carrier member with the stationary die carrier member.
An example of such a guide configuration is disclosed in the German Gebrauchsmuster (Utility Model) No. G 81 01 313 which suggests a push-type die closing unit, where the movable die carrier frame is guided on four horizontal tie rods and pushed closed by a single centrally mounted power cylinder. The tie rods extend between a stationary die carrier plate and a likewise stationary cylinder head plate, thus forming a rigid stationary assembly.
The machine base which carries this die closing unit has arranged on its longitudinal upper edges two guide rails which cooperate with guide shoes of the movable die carrier member. The guide shoes are of angular outline, so as to simultaneously engage a horizontal surface of each guide rail, for weight-supporting purposes, and a vertical surface of each guide rail, for lateral support and centering purposes.
The stationary die carrier plate and cylinder head plate are mounted on opposite extremities of the guide rails. In order to obtain the desired weight-supporting and centering action of the guide rails in relation to the movable die carrier member, the die closing unit must be positioned in relation to the machine base in a painstaking and time-consuming alignment procedure. The accuracy of this alignment thus depends on the skill and reliability of the assembly personnel. Consequently, it is virtually always necessary to verify the alignment accuracy at least once, in order to ascertain the absence of human error, experience having shown that even a minute misalignment will have noticeable negative effects on the guidance of the movable die carrier frame.
One negative effect of misalignment between the guide rails and the stationary plates is a marked increase in wear of the guide members. The achievement of precision-alignment is particularly important, when the guide members are of the rolling type, using a row of recirculating bearing rollers which are guided in grooves of a roller bogie, so that a plurality of rollers engage each guide surface of a guide rail. These roller bogies are highly sensitive to any misalignment.
The achievement of a high degree of accuracy and dimensional consistency over the entire production run of a particular injection-molded part depends to a great extent on the precision with which the movable die carrier member is guided in relation to the stationary die carrier member, for an absolutely parallel alignment of the two die halves. It has therefore become an important objective to avoid any deflection of the tie rods which may result from misalignment or misadjustment of the guide engagement between the movable die carrier frame and the guide rails of the machine base.
Lastly, it has also been found that a certain measure of distortion in the die closing unit is produced by the tie rods themselves, as they undergo elongation, when the full closing pressure is applied to the injection molding die by the power cylinder. This pressure exerts itself on the tie rods as a tensile stress which, due to the mounting attachment of the stationary die carrier plate and the cylinder head plate on the machine base, manifests itself in a greater elongation of the upper two tie rods as compared to the lower two tie rods.